Titanium oxide semiconductor photocatalytic system appears to be the most attractive for environmental purification of textile wastewater. The only drawback of TiO₂ semiconductor is that it absorbs a small portion of solar spectrum in the UV region (band gap energy of anatase TiO₂ is 3.2 eV and 3.0 eV for rutile TiO₂). Therefore, modification of TiO₂ photocatalysts for dyes degradation using visible light is a demanding area of research.

Three main aspects were studied: (1) to prepare various cadmium doped TiO₂ by doctor blade technique, the percent of doping was Cd:Ti=0.1, 0.5, 1, 3 respectively 5 at % and the cadmium precursors were acetate, nitrate respectively chloride; (2) to characterize the catalysts by surface area and porosity, X-ray diffraction (XRD) and atomic force microscopy (AFM), (3) to evaluate the kinetic studies for evolution of dyes concentration (M) under the following conditions: (a) UV irradiation without TiO₂ (photolysis), (b) in the dark with TiO₂ (adsorption) and (c) UV irradiation with Cd doped TiO₂ (photocatalysis/photodegradation). The kinetic of adsorption and degradation of Methyl orange and Methylene blue has been investigated and compared using different concentrations of dyes in range: 0.05-0.0125 mM. The kinetic of degradation of dyes fits well with the Langmuir–Hinshelwood model below a dye concentration of about 0.0125 mM. At higher dyes concentration the important absorption of light by the dyes was suggested to be at the origin of the decrease of degradation. No correlation was observed between adsorption isotherm in the dark and the degradation kinetics. However, in the textile wastewater, often a mixture of dyes and heavy metal compounds are present, so our objectives in the future will be to study the kinetics of degradation of a mixture of dyes and cadmium ions.

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